Paint Calculator

What this calculator does

Give it a room — either length × width × height or a wall area you already measured — and it returns the paint to buy, not just a theoretical volume. Doors and windows are deducted at editable standard sizes (~20 sq ft per door, ~15 sq ft per window), coats multiply the area rather than being guessed at, and the coverage rate is yours to set: presets for standard interior (~350 sq ft/gal), high-hide premium (~400), textured or porous surfaces (~300) and primer (~200–300), or any custom figure off your can’s label. The ceiling toggle adds length × width as its own line; trim prices separately by length × width, because ceiling white and trim enamel are different cans than your wall color.

The answer comes back as a shopping plan: exact need next to what you’d actually buy — gallons and quarts in imperial, 1 / 2.5 / 5 / 10 L tins in metric — priced at the per-unit cost you enter. Switching between imperial and metric converts your inputs honestly (12 ft ↔ 3.66 m, 350 sq ft/gal ↔ 8.6 m²/L), and the whole thing runs in this tab: nothing is sent anywhere.

The math, in the open

net walls = 2 × (L + W) × H − (doors × 20 + windows × 15)  ·  volume = net area × coats ÷ coverage

• Deductions are real: a standard door is ~20 sq ft, a standard window ~15 sq ft — both editable, because French doors and picture windows are not standard. If openings ever exceed the wall area the result clamps to zero instead of going negative, and it tells you.
• Coats multiply area, not vibes: two coats at 350 sq ft/gal need twice the paint of one — the plan shows the coats-adjusted total, which is where most mental math goes wrong.
• Rounding is to retail units: quarts and gallons (imperial) or the common 1 / 2.5 / 5 / 10 L tin sizes (metric), minimizing leftover. Cost is priced on that buy plan — the cans you pay for — not the neat theoretical number.
• What’s deliberately not modeled: primer as a separate coat (select the primer preset to price it), a 10% waste buffer (the round-up to cans is the buffer), accent walls in a second color (do that math here), and textured or double-height surfaces — lower the coverage rate or measure the odd shapes first.

Worked example — a 12 × 14 ft room, 9 ft ceilings

Gross walls: 2 × (12 + 14) × 9 = 468 sq ft. Two doors and three windows: 2×20 + 3×15 = 85 sq ft deducted → 383 sq ft net. Two coats at 350 sq ft/gal = 2.19 gal exact → buy 2 gal + 1 qt, about $101 at $45/gal. Tick the ceiling and 168 sq ft more wants another gallon; add 52 ft of 3.5-inch baseboard and a quart of enamel covers it twice over.

Go deeper

• Paint coverage rates by type — why the label’s 400 sq ft/gal is an opening bid, and what each paint type really covers.
• How to measure a room for paint — L-shapes, vaults, stairwells and open plans, wall by wall.
• When you actually need primer — the five cases where it matters and the many where it doesn’t.
• Paint sheen guide — flat to high gloss: where each belongs and what it does to coverage.
• Accent wall math — one wall, one color: why the answer is usually “buy a quart, not a gallon.”
• Why you always buy too much paint — the seven classic overbuy mistakes and their fixes.

Frequently asked questions

How many square feet does a gallon of paint cover?

Labels typically claim ~400 sq ft per gallon for interior wall paint; 350 is the safer real-world figure for smooth drywall, and textured or porous surfaces can drop to ~250–300. Primer runs lower, ~200–300 sq ft/gal. In metric the same claims read ~10–12 m²/L for wall paint and ~5–7 m²/L for primer. The full table by paint type is in coverage rates by paint type.

How much paint do I need for a 12 × 12 ft room?

With 9 ft ceilings the walls are 432 sq ft gross; a typical two doors and two windows (~70 sq ft) leave ~362 sq ft net. Two coats at 350 sq ft/gal is ~2.1 gal — buy 2 gallons + 1 quart. One coat of a same-color refresh would be ~1 gal. Run your own numbers above — ceiling and trim change the shopping list, not the wall paint.

Should I really deduct doors and windows?

Yes — the ~20 sq ft per door and ~15 sq ft per window add up fast in small rooms. On a 10 × 10 ft room they can be 15% of the wall area, the difference between needing a second gallon and not. Deduct them, then keep a sensible margin: buying the next unit up is still cheaper than a second trip to the store.

Does the calculator include ceiling and trim?

Separately, on purpose. Ceiling area (length × width) prices out at your coverage rate just like walls, and trim is length × average width — but both are usually different products (flat ceiling white, trim enamel) bought in their own cans. Each gets its own line in the shopping plan instead of being silently merged into the wall number.

Gallons or litres — which does it use?

Both, honestly converted. Imperial mode works in feet, sq ft and US liquid gallons (1 gal = 3.785 L); metric mode works in metres, m² and litres. Switching converts your typed numbers exactly — 12 ft becomes 3.66 m — and swaps the coverage presets to the equivalent local claims (US ~350–400 sq ft/gal, UK ~10–12 m²/L). UK cans come in litres, so metric mode plans 1 / 2.5 / 5 / 10 L tins.

Why does it say 'nothing to paint'?

Your door and window deductions meet or exceed the wall area — mathematically the net wall is zero, so the result clamps instead of going negative. In a real room that's nearly impossible (openings are rarely more than ~25% of walls), so double-check the sizes you entered. The reveals and returns around openings do get painted in practice.

Is the cost estimate what I'll actually pay?

It prices the buy plan — the gallons, quarts or litres you'd actually purchase — at the per-unit prices you type, so it's a materials estimate for paint only. It doesn't include primer, sundries (tape, rollers, drop cloths), or sales tax, and paint prices vary wildly by brand and sheen. Treat it as the checkout total for cans, nothing more.